This invention pertains generally to neutron flux radiation detectors and more particularly to ultrasonic monitors that measure neutron flux radiation levels.
Generally, there are three categories of nuclear radiation detectors presently available in the art. The categories can be broadly classified respectively as solid state or semiconductor detectors, scintillation counters, and gas filled detectors such as ion chambers and proportional counters. In all cases, the electrical output signal is basically a current which flows for a brief period of time; i.e., an impulse charge, the dimensions of which is the product of current and time. The area of the impulse, the total charge generated by each ray or particle absorbed in the detector, is directly proportional to the absorbed energy. The duration of the output current varies with the type of detector and with the nature and energy of the radiation. The currents of the shortest duration, e.g., 0.1 to 10 nanoseconds, occur in plastic scintillators and narrow-depletion-depth semiconductor detectors and the longest current durations of approximately 0.1 to 5 microseconds occur in gas-filled detectors. Scintillators, such as sodium iodide and cesium iodide exhibit outputs with intermediate current durations of approximately 0.75 to 1.5 microseconds.
In semiconductor detectors, the energy conversion factors are W = 2.8 and 3.5 electron-volts per electronic charge for germanium and silicon, respectively. No charge multiplication occurs in semiconductor detectors. In scintillation counter dectectors, the effective W is 3 to 30 Kiloelectron-volts per electron with multiplication factors as high as 10.sup.8 occurring in the photomultiplier. In gas-filled detectors, the conversion factor is 25 to 36 electron-volts per ion pair. No multiplication occurs in ion chambers, but multiplication factors of as high as 10.sup.4 are common for proportional counter detectors. Thus, depending on the detector and the energy of the ambient radiation being monitored, signal strengths may vary from those at amplifier noise levels to amplitudes at which noise is negligible. Accordingly, the energy, the desired resolution and the expected counting rate of the radiation to be detected, will determine the choice of the detector.
Generally, the biasing circuits employed for both semidonductor and gas-filled detectors are quite similar. The detector leakage current flowing through the resistor commonly in series with the biasing voltage will cause a loss of biasing voltage. For room temperature Surface Barrier Detectors, this current is typically between 0.1 and 1 microampere. In gas counter detectors and cooled Ge or Si detectors, the leakage current is usually negligible. Additional bias voltage loss is generally caused by the average signal current flowing through the resistor which equals nq = (nE/W) amperes, where q is the charge of an electron, n is the counting rate, E is the energy absorbed in the detector, and W is the energy conversion factor in electron voltages per ion pair.
Both leakage current and the bias resistor introduce noise. The leakage current introduces shot noise while the resistor introduces what is known as Johnson noise. Although the mechanism of noise generation is different for each case, their respective noise contributions are indistinguishable.
Another potential noise source is a capacitor which is normally used to AC couple the detector to the preamplifier while isolating the bias voltage from the input. The voltage rating of the capacitor must satisfy the highest detector bias used, and the capacitor must be noise free at that voltage. Low leakage capacitors such as mylar or high quality ceramic dielectric capacitors are generally recommended.
Both semiconductor and gas-filled nuclear radiation detectors require the use of a DC high voltage bias for operation. Also, the nature of their output signals is such that relatively little can be done to improve their signal-to-noise ratio when they are operated in severe industrial environments. A nuclear radiation detector that does not require a DC bias voltage, and whose output signal frequency is adjustable for an optimum signal-to-noise ratio, will supply substantial improvement in detector responses obtained in the severe operating environment encountered in most environment nuclear applications.